While wind and solar photovoltaic technologies are crucial to meeting our future energy needs, the inherent variability in both can raise concerns. "The Power of Transformation: Wind, Sun and the Economics of Flexible Power Systems" released by the IEA in 2014 addresses the additional costs that might arise from increased adoption and proposes strategies to develop a flexible system in the long term.
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In 2010 the IEA released two Technology Roadmaps: the solar Photovoltaic (PV) and Concentrating Solar Power (CSP). The reports detail what milestones must be reached for Photovoltaic and Concentrating Solar technologies to provide up to 9 000 TWh of power by 2050.
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About solar photovoltaics

Definitions

The basic building block of a PV system is the PV cell, which is a semiconductor device that converts solar energy into direct-current electricity. PV cells are interconnected to form a PV module, typically up to 50 to 200 Watts. The PV modules, combined with a set of additional application-dependent system components (e.g. inverters, batteries, electrical components, and mounting systems), form a PV system. PV systems are highly modular; i.e. modules can be linked together to provide power ranging from a few watts to hundreds of megawatts.

The most established solar PV technologies are crystalline silicon-based systems. Thin-film modules, which can also consist of non-silicon semiconductor material, represent about 10% of the global market. Concentrating PV, where sunlight is focused onto a smaller area has just entered full market deployment. Concentrating PV cells have very high efficiencies of up to 40% - but only with respect to direct normal irradiance. Other technologies, such as organic PV cells, are still in the research phase.

Solar PV combines two advantages. On the one hand, module manufacturing can be done in large plants, which allows for economies of scale. On the other hand, PV is a very modular technology. It can be deployed in very small quantities at a time. This quality allows for a wide range of applications. Systems can be very small, such as in calculators, up to utility-scale power generation facilities.

Compared to concentrating solar power (CSP) and CPV, non-concentrating (“1-sun”) PV has the advantage that it uses not only direct sunlight but also the diffuse component of sunlight, i.e. solar PV produces power even if the sky is not completely clear. This capability allows the effective deployment in many more regions in the world than for CSP.

Because PV generates power from sunlight, power output is limited to times when the sun is shining. However, as IEA analysis under the Grid Integration of Vraiable Renewables (GIVAR) project has highlighted, a number of options (demand response, flexible generation, grid infrastructure, storage) exist to cost-effectively deal with this challenge.

About concentrating solar power

Definitions

Concentrating solar power (CSP) devices concentrate energy from the sun’s rays to heat a receiver to high temperatures. This heat is then transformed into electricity – solar thermal electricity (STE).

Concentrating solar thermal power and solar fuels technologies produce electricity and possibly other energy carriers (“fuels”) by concentrating solar radiation to heat various materials to high temperatures. A concentrating solar power (CSP) plant comprises a field of solar collectors, receivers, and a power block, where the heat collected in the solar field is transformed into mechanical energy, then electricity. In between, the system must include one or several heat transfer or working fluids, possibly heat storage devices and/or back-up/hybridisation systems with some combustible fuel. A cooling system, wet or dry, completes the description of the plant (IEA, 2010d). CSP plants come in four different versions: parabolic trough, linear Fresnel, tower and parabolic dish systems.

About solar heating & cooling

Solar heating:

A variety of technologies exists to capture solar radiation and convert it into heat for a wide number of applications. Several solar heating technologies are already mature and can be competitive in certain regions of the world in applications such as domestic hot water heating and swimming pool heating.

The most mature technology, the solar domestic hot water system, has a long history but was first deployed on a large scale in the 1960s in countries such as Australia, Japan and Israel. Since then some markets have shown strong increased deployment as a result of the introduction of long-term subsidy schemes or solar obligations (e.g. subsidies in Austria and Germany, and solar obligations in Israel and Spain) or as a result of solar hot water systems’ competitive advantages over alternative technologies (e.g. China, Cyprus). Over the past 15 years, China’s economic development has spurred the market for solar hot water heating in terms of both system component manufacture and end-use demand.

Solar thermal heating installations have been growing considerably around the globe in the last decade. Global installed solar thermal capacity was 268 GWth in 2012, including district heating and industry installations. Global solar thermal energy use for heat in buildings grew at 12% per year on average, and reached 0.7 EJ in 2011. By far, the fastest-growing market for solar thermal energy for heat worldwide is China, where installed capacity increased more than seven-fold between 2000 and 2011, to 152 GWth in 2011. The final solar thermal energy use for heat in buildings in China has been growing accordingly, from 0.04 EJ in 2000 to 0.46 EJ in 2011. Almost all of this this was for hot water provision.

Solar cooling:

Solar thermal district cooling networks could have strong potential for the enhanced use of renewable heat for cooling, as availability of the solar resource usually correlates to the cooling demand in buildings. Estimates indicate around 1 000 solar cooling systems installed worldwide at the end of 2012, with 80% of these installations found in Europe (mainly Spain, Germany and Italy). While solar cooling remains a niche market, the market has grown considerably in recent years.